The true core of GitHub (and GitLab) is Git itself. So both are open core where it really matters. Because both GitHub's and GitLab's essential operations are a

The true core of GitHub (and GitLab) is Git itself. So both are open core where it really matters. Because both GitHub's and GitLab's essential operations are all done over the Git core, neither code and code history hosted on either system are locked in. Where GitHub has slightly more lock-in and is less open than GitLab is its Issues system; One can only extract the data (via API) for import into an alternative system, whereas with GitLab you could run your own copy of the open source code (but you'd still have to do the data export-import as far as I can tell).